The efficiency of mixtures of ionic liquids (ILs) and molecular solvents in cellulose dissolution and derivatization depends on the structures of both components. We investigated the ILs 1-(1-butyl)-3-methylimidazolium acetate (C 4 MeImAc) and 1-(2-methoxyethyl)-3-methylimidazolium acetate (C 3 OMeImAc) and their solutions in dimethyl sulfoxide, DMSO, to assess the effect of presence of an ether linkage in the IL side-chain. Surprisingly, C 4 MeImAc-DMSO was more efficient than C 3 OMeImAc-DMSO for the dissolution and acylation of cellulose. We investigated both solvents using rheology, NMR spectroscopy, and solvatochromism. Mixtures of C 3 OMeImAc-DMSO are more viscous, less basic, and form weaker hydrogen bonds with cellobiose than C 4 MeImAc-DMSO. We attribute the lower efficiency of C 3 OMeImAc to “deactivation” of the ether oxygen and C2–H of the imidazolium ring due to intramolecular hydrogen bonding. Using the corresponding ILs with C2–CH 3 instead of C2–H, namely, 1-butyl-2,3-dimethylimidazolium acetate (C 4 Me 2 ImAc) and 1-(2-methoxyethyl)-2,3-dimethylimidazolium acetate (C 3 OMe 2 ImAc) increased the concentration of dissolved cellulose; without noticeable effect on biopolymer reactivity.